Curable adhesives are useful in industry. An example of curable adhesives is the type described in U.S. Pat. No. 5,252,694. Specific applications for pressure sensitive adhesives, having sufficient clarity, involve the preparation of optical elements. See, for example, U.S. Pat. Nos. 6,180,200 and 5,897,727. Curable pressure sensitive adhesives can be useful and preferred due to their ability to be applied to substrates while the adhesive is tacky like a pressure sensitive adhesive, and to subsequently cure the adhesive to form a secure structural adhesive bond.
While a variety of curable adhesives have been prepared from countless combinations and mixtures of ingredients including various polymers, e.g., acrylates, (meth)acrylates; epoxies; crosslinkers; other heat and cationically curable polymers; and heat or radiation sensitive initiators; not all such compositions are suitable for use, especially in optical applications where clarity and stability are required. A useful adhesive for optical applications should be optically clear, and should also maintain that clarity over the life of the product in which it is used. The adhesive should withstand conditions present during manufacturing of optical elements and during use of such elements without losing a substantial amount of clarity. This property involves the adhesive being stable on its own, and also being stable and compatible when used with other elements in an optical element or optical product.
Optical elements include not only adhesives but other components bonded together by the adhesive. The adhesives are used, for example, to bond materials to polymeric materials such as polyesters; to bond materials to rigid materials such as rigid polycarbonate, polymethyl methacrylate, or glass; to bond materials to polarizer layers; etc.
Often, any of these components of an optical element, or other adjacent components, can adversely affect the stability, clarity, bond strength, or other performance property of an adhesive in the same optical element. Polycarbonates, for example, are known to outgas, producing bubbles or partial or full delamination at the adhesive bond between the polycarbonate and another layer of an optical element. Bubbling and delamination can be particularly common when the outgassing layer is bonded to another layer or laminate that exhibits low vapor transmissivity. Bubbles and delamination can affect clarity and integrity of the optical element, and must be avoided. It can be preferred, therefore, in these and other settings, that an adhesive be stable and not delaminate, bubble, or lose its clarity or integrity during use.
As another matter, other optical materials can be particularly fragile, and may, for example, exist in the form of a fragile film. The fragile film can be a material that is not thermally stable, is not dimensionally stable, is water or humidity sensitive, etc. Such a material may shrink or otherwise degrade when exposed to a high temperature or high humidity condition. It can be difficult to include these fragile materials in optical elements, due to such instability. An example of a type of fragile material is polarizer of the type known as a “KE polarizer,” which, due to their thinness, may shrink when exposed to a certain minimum temperature and humidity. It can often be useful and even advantageous for a variety of reasons such as cost or performance, to use such fragile materials in a larger optical element. Thus, it would be useful to find a way to maintain the stability of fragile materials so they can be used in useful products, including optical elements.